Acid-steam sterilization

ABSTRACT

A method for sterilization of containers. A plurality of open containers are heated by superheated steam to above 212*F. in a first heating chamber. The heated containers are then sterilized with an acid-steam mixture at a temperature of about 240* to 300*F. in a second heating chamber. The containers are filled and sealed in a sterile air-steam atmosphere immediately upon leaving said second heating chamber.

0 United States Patent 11 1 1111 3,839,843 Stewart, Jr. Oct. 8, 1974ACID-STEAM STERILIZATION 2,660,513 11/1953 Ball 21/79 x 2,685,520 8/1954Martin 53/25 X [75] Inventorfirs Stewart Commg, 2,768,487 10/1956 Day etal. 53 11 3 2,950,587 8/1960 Harmon et al. 53/22 R 3,042,533 7/1962McConnell et a1. 21/57 X [73] Ass'gnee at? Stewart Commg 3,139,3236/1964 Shields et a1 21 57 3,180,740 4/1965 Martin 99/182 [22] Filed:Jan. 23, 1973 3,576,594 4/1971 Knetemann et a1 21 57 [21] Appl' 325971Primary Examiner-Travis S. McGehee Related US. Application DataAssistant Examiner-Horace M. Culver [63] Continuation of Ser. No.70,803, Sept. 9, 1970, AtfomeyAgemor Firm wegnerstenmanMccord abandoned.Wiles & Wood [52] US. Cl 53/37, 53/22 R, 21/57, [57] ABSTRACT 51 I t C]4 2 A method for sterilization of containers. A plurality of d l l 8open containers are heated by superheated steam to l em 1 1 1/ above212F. in a first heating chamber. The heated 21/79 99/182 containers arethen sterilized with an acid-steam mix- 56 R f d ture at a temperatureof about 240 to 300F. in a sec- 1 e erences 0nd heating chamber. Thecontainers are filled and UNITED STATES PATENTS sealed in a sterileair-steam atmosphere immediately 1,593,121 7/ 1926 Gray 21 /79 uponleaving said second heating chamber. 1,754,251 4/1930 Yates 21/571,889,629 11/1932 Biihmer et al. 53/22 R 1 Clam" 1 Drawmg PRE-HEATINGCHAMBER FOR CONTAINERS (SUPERHEATED STEAM MORE THAN 212) PRE-HEATINGCHAMBER FOR COVERS (SUPERHEATED STEAM MORE THAN 212) ACID-STEAMACID-STEAM 3 STERILIZING sremuzme 4 CHAMBER CHAMBER 5 6 a 1 I STERILIZEDFlLLlNG FILLING AND SEALING MEANS QQP Q MATERIAL (AlR-STEAM ATMOSPHERE)CONTAINERS) ACID-STEAM STERILIZATION This is a continuation ofapplication Ser. No. 70,803, filed Sept. 9, 1970, now abandoned.

BACKGROUND OF THE DISCLOSURE The instant invention is directed to amethod for sterilizing containers. More specifically, the instantinvention is directed to a method for sterilizing containers in a twostage operation in which the second stage includes exposing thecontainers to an acid-steam mixture. Most specifically, the instantinvention is directed to a method for sterilizing containers in a twostage operation in which the containers are first heated to atemperature of above 212F. by superheated steam and thereaftersterilized in a second heating step in which the containers are exposedto an acid-steam sterilizing mixture.

In the prior art, various methods have been advanced for sterilizingcontainers. Included among these methods is the process of sterilizationby the use of acidified steam. An atmosphere of acid-steam vapordestroys microorganisms at lower temperatures then an atmosphere ofsteam only. It is hypothesized that acid either weakens all or destroysmany of the microorganisms present so that a lower temperature isrequired to completely destroy the remainder of the microorganismspresent. Thus, the complete destruction of all microorganisms, orsterilization, can be accomplished at lower temperatures in the presenceof acid. Therefore, steam sterilization is accomplished at lowertemperatures, with all the attendant advantages thereof, when a smallpercentage of acid is added to the steam.

The method of acid-steam sterilization has been advanced, in the priorart, for sterilization of containers to be used in the packaging of asterile product. Although the prior art methods provide advantages oversteam sterilization methods as described above, they are subject tocertain disadvantages which make their use less attractive.

In one process suggested by Shields et al (US. Pat. No. 3,139,323) anacid at a concentration of 0.36 to 3.6 percent by weight, in anacid-steam mixture, is used to sterilize containers. The temperature ofthe mixture is stated to be 150 to 212F. The inventors suggest thatsubjecting articles to the mixture for a period of seconds to 10 minutesis enough to sterilize the article. Although low temperaturesterilization is an important advantage of acidified steam processes, atthese relatively low temperatures a film of the acid remains on thesurface of the container. Thus, the inventor admits that it isfrequently necessary to rinse or heat vaporize the container prior tofilling. Such an additional step is costly. Moreover, if heatvaporization is used, and as suggested by the inventor hydrochloric acidis employed as the acid, a constant boiling point mixture results. Atatmospheric pressure, a constant boiling point hydrochloric acid-watermixture forms. This mixture boils at a temperature of 227.5F. with a BC]concentration of 20.2 percent. At this high acid concentration corrosionof the container often results.

7 A second process suggested in the prior art, the patent to Martin (US.Pat. No. 3,180,740), describes a second acid-steam sterilizationprocess. In it, Martin suggests wetting the container surface with anacid and then heating with steam at a temperature at a range of 212 to270F. Thus, Martin overcomes the low temperature disadvantage of theShields et al method in so far as the Martin process requires anacid-steam mixture temperature in excess of 212F. for articlesterilization. Although the Martin sterilization method occurs at highertemperatures it does not solve the corro sion problem discussed above.As the aqueous acid solution on the container surfaces is heated, theacid solution on the container surface is concentrated due to theformation of a constant boiling point mixture. As stated above, commonlyused hydrochloric acid boils at the high acid concentration of 20.2percent. This, of course, can result in corrosion of the containersurfaces. If rinsing is substituted for acid boiling, the additionalcost of rinsing similarly makes the Martin process less attractive.

Thus, the prior art methods employing acid-steam for sterilization aresubject to the problem of the presence of acid solution on the containersurfaces. This, in turn, results in corrosion of the containers, oralternatively to an additional rinsing step. This and otherdisadvantages are overcome by the method of the instant invention whichis described in greater detail hereinafter.

SUMMARY OF THE INVENTION The instant invention is directed to a processfor sterilization of containers by acid-steam contact in which no acidis condensed. Therefore, the instant invention is directed to a processof acid-steam sterilization in which the problem of container surfacecorrosion does not arise. This is accomplished by preheating thecontainers above the temperature at which the acid-steam mixturecondenses prior to exposure of the containers to said acid-steam mixtureatmosphere.

In accordance with the instant invention, a method is provided forsterilizing containers in which a plurality of containers are heated bysuperheated steam to a temperature in excess of 212F. in a first heatingchamber. The heated containers are then sterilized in an acid-steamatmosphere at a temperature in excess of the acidsteam condensationtemperature in a second heating chamber. They are thereafter immediatelyfilled and sealed in a sterile atmosphere.

BRIEF DESCRIPTION OF THE DRAWING The instant invention may be betterunderstood by the accompanying drawing which is a block diagram of apreferred embodiment of the method of the instant invention.

DETAILED DESCRIPTION Turning to FIG. 1 in detail, the pre-heating orfirst heating stage is directed in steps 1 and 2. Step I illustrates thepre-heating of the containers. It should be understood that thecontainers include uncovered cans, receptacles and the like. Thecontainers are heated in the first heating stage 1 to a temperature inexcess of 212F. More preferably, the containers are heated in stage 1 toa temperature in the range of about 225 to 240F. In a preferredembodiment the heating medium is superheated steam. Steam is a preferredmethod for pre-heating of containers. As between saturated steam andsuperheated steam, superheated steam is preferred since it permits theattainment of higher temperatures at lower pressures. This, in turn, notonly insures the structural integrity of the containers, but alsoeliminates the necessity of a pressurized chamber, which is required iftemperatures appreciably in excess of 212F. are required and saturatedsteam is used. It

should be appreciated that if a chamber designed to withstand higherpressures is available and the containers themselves are of sufficientstrength to withstand pressures in excess of atmospheric saturated steammay be used. It should be further appreciated that other heating means,other than steam, may be substituted as long as the containers leave thefirst heating stage at a temperature of more than 212F. and preferably225F. to 240F.

In one preferred embodiment, the containers are disposed on a containermoving means such as a porous conveyor belt in an upside downconfiguration. That is, the containers are placed on the porous beltwith open end rims of the containers contacting the belt. In this way,any condensate that may form in the container can drain to the open endof the container and drop to the bottom of the heating chamber throughthe pores of the porous belt.

In the preferred embodiment wherein the containers are disposed onporous belt and are heated by steam, the steam, which is preferablysuperheated, enters the chamber so as to heat the containers bycountercurrent convective heat transfer. Thus, the steam enters thefirst heating stage chamber at the container outlet end thereof andleaves through a steam exiting means located at the container inlet endof the chamber. The expended steam, which exits through the steamexiting means may be reheated and recycled for reuse. Alternatively,fresh steam may be continually employed.

The heated containers leave the pre-heating stage 1 at a temperature ofat least approximately 212F. This is insured by employing countercurrentheat transfer means in stage 16. As the containers leave the firstheating chamber, they are heated by steam, just entering the chamber.The already partially heated containers are thus heated to a temperatureapproaching that of the incoming steam which enters at a temperature inexcess of 212F'.

The lids, which are to be disposed on and sealed to the open containers,as will be described in greater detail hereinafter, are also treated ina first heating or preheating stage. This is depicted in the drawing asheating stage 2. In stage 2, the lids, which may include a sealingmembrane, as well as an outer cover, are treated in a manner exactlyanalogous to the treatment afforded the containers in heating stage 1.It should be understood that when the word lid is used it is descriptiveof a membrane also. Thus, in a preferred embodiment a plurality of lidsare heated by the countercurrents convective heat transfer withsuperheated steam as the heating medium. Again, saturated steam or evenheating means other than steam may be substituted. Since, thepre-heating treatment of the lids is the same as the pre-heatingtreatment of the containers, they may be treated in the same chamber. Inthis case, a plurality of containers are first heated, followed by aplurality of lids. Alternatively, a container followed by a lid may bedisposed on the porous conveyor belt. Of course, two separate chambers,one for containers and one for lids, as suggested in the drawings asstages 1 and 2, respectively, may likewise be employed.

Returning now to the treatment of the containers, they leave thepre-heating stage 1 at a temperature of at least 212F. and preferably ata temperature of about 225F. to 240F. The heated containers immediatelyenter an acid-steam sterilizing or second heating stage 3. Thesterilizing stage 3 comprises another chamber,

which in a preferred embodiment, is again provided with a containermoving means. Typical of such means is again a porous conveyor belt. Thecontainers are again preferably disposed on the belt in an invertedconfiguration with opened end of the containers in contact with thebelt. This positioning is a redundant safety procedure, as will bedescribed in greater detail hereinafter. The containers on the belt areheated by an acid-steam gaseous mixture which flows countercun rently tothe movement of the plurality of containers stationed on the movingporous belt. Thus, the advantages of countercurrent convective heattransfer are again derived.

The unique acid-steam gaseous mixture provides the sterilizing means inthe method of the instant invention. A small amountof acid, which in apreferred embodiment is hydrochloric acid or acetic acid is added tosteam to provide the sterilizing mixture. It should be appreciated thatother acids may be substituted for the acids mentioned above, butusually hydrochloric or acetic acid is employed. Independent of the acidutilized, enough acid is added to the steam so that the gaseous mixturehas a pH in the range of about 2.8 to 3.2, as measured in thecondensate, after the steam has left the chamber and has been cooled. Inthe preferred embodiments wherein hydrochloric or asetic acid is used,0.06 percent down to 0.023 percent by weight of by drochloric acid, or0.90 percent down to 0.19 percent by weight of acetic acid is added tosteam to provide a pH in the preferred range of 2.8 to 3.2,respectively. It should be understood that the pH of the gaseous mixturemay be outside the range indicated. However, it has been found thatsterilization, is optimized under these conditions.

The steam employed is preferably superheated for the reasons givenabove. Thus, it is possible to use a saturated steam-acid mixture if thechamber and containers are properly designed to withstand the increasedpressures of saturated steam. However, in the sterilizing stage 3, thereis an additional reason for employing superheated steam. This reason,which will be discussed below, makes it even more preferable to employan acid-superheated steam mixture.

Whether the steam is saturated or superheated, the acid-steam mixtureenters the sterilizing chamber at a temperature range in excess of theacid-steam condensation temperature. Preferably, this temperature rangesfrom about 240 to 300F. At these temperatures, and in the presence ofacid, and in the concentration range noted above, the containers aresterilized. It has been found that a residence time of 10 to 25 minutesis required to sterilize containers in sterilizing stage 3 at the lowerend of the sterilizing temperature range, that is, about 240F. At theupper sterilizing temperature range of approximately 300F., theresidence time is noticeably decreased to a period of about 1 to 5minutes and preferably 3.5 minutes. Thus, the containers have aresidence time of about 1 to 25 minutes, when an acid-steam mixturehaving a condensate pH of about 2.8 to 3.2 and a temperature of about240 to 300F. is used as the sterilizing medium. The time-temperaturecombination chosen by the sterilizer is usually a function of containermaterial. Thus, a metal container will usually withstand highertemperatures then a plastic container. Therefore, metal containers areusually sterilized at higher temperatures for shorter times than plasticcontainers.

The containers exiting the first heating stage 1, as stated above, areheated to a temperature of at least 2 1 21 Hence, there is nopossibility that cold container surfaces will condense any of theacid-steam mixture in sterilizing stage 3 at atmospheric pressure.Acid-steam condensation can cause container corrosion, as discussedabove. Thus, the method of the instant invention eliminates the problemof corrosion from acidsteam container sterilization. It should beappreciated, from the above, that in order to insure noncondensationduring sterilization that the entrance temperature of the containersinto the sterilization stage 3 must be greater than 212F. if theacid-steam mixture pressure is higher than atmospheric. Since thepreferred entrance temperature is about 225 to 240F. this should not bea problem even if the steam is only slightly superheated. Of course, themore superheated the steam in the acid-steam mixture the more remote thepossibility of condensation.

A final precaution, mentioned above, is also employed to guard againstthe remote possibility of acid condensation. That is the disposition ofthe open containers in an upside down configuration on the belt. If anycondensation does occur, it is probable that the condensate will run outthe open end prior to the container leaving the chamber.

An analogous sterilizing step occurs in sterilizing stage 4. In stage 4the lids are sterilized under the same conditions as discussed above inregard to the operation of stage 3. The only distinguishing featurebetween lid sterilization and container sterilization is that the lidscannot be disposed on the porous belt in an upside down position. Apreferred design provides for designing the conveyor belt so that thelids are disposed in an upright position. This preferred configurationnot only permits condensate removal but better exposes the lid surfacesto the sterilizing acid-steam mixture. Because of the similarity ofoperating conditions in stages 3 and 4, in one preferred embodiment,sterilization of the lids and the containers occurs in the same chamber.The use of a single chamber is usually preferred in those cases wherethe operation of the pre-heating stages 1 and 2 is accomplished in asingle pre-heating chamber.

Following sterilization in stages 3 and 4, which may occur in one or twochambers, the sterilized lids and containers are placed in a chamberwhich constitutes the filling and sealing stage 6. Previously sterilizedfilling material 5 is placed into the open sterilized containers, whilethe containers are in the filling and sealing stage 6, by filling meanswell known in the art. After filling of each container to a point belowthe top of the open end, a lid is placed on the open end and sealedthereto. The filling and sealing chamber is presterilized and maintainedsterile by a sterile steam-air atmosphere maintained at a pressure ofabout 2 to 6 inches of water above atmospheric pressure. This atmospherepermits vacuum sealing. The steam-air atmosphere fills the headspacebetween the top surface of the sterile material and the lid. After thesealed containers leave the filling and sealing stage 6, they arecooled. This results in condensation of the steam in the headspace andthe resultant formation of a vacuum. The final product, in sealedcontainers is designated in the drawing by reference numeral 8.

It should be appreciated that the above-described preferred embodimentof the method of the instant invention is meant to be illustrative only.The foregoing specification and drawing will make apparent variousmodifications which are within the contemplation of the instantinvention. Thus, the scope of the invention should be limited only bythe appended claims.

What is claimed is:

l. A method for aseptic canning of a sterile product comprising thesteps, in seriatim, of:

a. first preheating a plurality of open containers and sealing lids to atemperature of about 225 to 240F. under substantially atmosphericpressure, and then b. sterilizing said heated containers and lids byexposing them to an acid-steam mixture maintained at substantiallyatmospheric pressure and at a temperature in the range of 240 to 300F.for a period of about 1 to 25 minutes, said mixture having a pH of about2.8 to 3.2 as measured in the condensate of said acid-steam mixture; andthereafter canning said sterile product in a sterile air-steamatmosphere maintained at a pressure 2 to 6 inches of water aboveatmospheric pressure by filling said open containers with said sterileproduct, covering said containers with said lids and thereafter sealingsaid lids to said containers.

1. A method for aseptic canning of a sterile product comprising thesteps, in seriatim, of: a. first preheating a plurality of opencontainers and sealing lids to a temperature of about 225* to 240*F.under substantially atmospheric pressure; and then b. sterilizing saidheated containers and lids by exposing them to an acid-steam mixturemaintained at substantially atmospheric pressure and at a temperature inthe range of 240* to 300*F. for a period of about 1 to 25 minutes, saidmixture having a pH of about 2.8 to 3.2 as measured in the condensate ofsaid acid-steam mixture; and thereafter c. canning said sterile productin a sterile air-steam atmosphere maintained at a pressure 2 to 6 inchesof water above atmospheric pressure by filling said open containers withsaid sterile product, covering said containers with said lids andthereafter sealing said lids to said containers.